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Women who have breast cancer would benefit from a PET scan that can show if there is any other cancer in their body, a new study has shown.

Researchers at the Ahmanson Biological Imaging Clinic at the University of California Los Angeles found that in 60 per cent of breast caner patients, a full body scan resulted in a reassessment of treatment. In 36 per cent of cases the scan showed the stage of the cancer's development had been misjudged.

"I think the PET scanner is going to be very useful," said Dr David Clark, from The Breast Clinic in Newcastle. "But it is in its fairly early days, certainly as far as breast cancer is concerned."

In the US trial, Dr Johannes Czernin and colleagues analysed PET scan results for 50 patients with known breast cancer.

PET - positron emission tomograpy - measures the body's metabolic activity. About 45 minutes before the scan, the patient is injected with a drug that emits signals which picked up by the scanner, then reinterpreted by a computer into an image of the body. Cancerous areas show up brightly because they are more metabolically active than non-cancerous cells.

The scans showed that some cancers were more serious than had been thought.

Before the scan 36 per cent of cancers were classified as stage IV, meaning the cancer had spread to an unrelated part of the body. After the scans, this proportion rose to 52 per cent because the scans found previously undetected metastases. In contrast, 8 per cent of patients had the stage of their cancer downgraded.

Mammograms are currently the best screening option for breast cancer but they are not completely reliable, said Dr Clark.

"We know from our own figures at The Breast Centre that with 15 per cent of people who have a cancer in their breast, it did not show up as a cancer on a mammogram."

PET scanning is relatively new in Australia. In all of New South Wales, for example, there is only one PET scanner - at Royal Prince Alfred Hospital in Sydney. The Federal Government is currently deciding where to allocate further scanners.

However women in Australia do have access to another form of accurate scanning, said Dr Clark. Most of his patients undergo scintimammography, a technique that uses a nuclear scanner to pick up a localised area of radioactivity.

His research on the technique, published in the Medical Journal of Australia in 1999, showed that it changed treatment decisions in five to eight per cent of patients.

"We found the scans were very helpful in that some lesions that appeared benign in a mammogram turned out to be cancerous," Dr Clark explained.

"Importantly, when we knew someone had a cancer in the breast, the scan sometimes showed they had two or three other smaller cancers not previously known about."

Breast cancer is the most common cancer in Australian women, affecting one in 12 women and causing more than 2,500 deaths every year.

If detected early, while still localised in the breast, chances of five-year survival are around 90 per cent. If it has already spread to distant parts of the body, the survival rate drops to 18 per cent.